You make a good point, it is a strange one. But in this context par is meant to mean, the average, of the usual quality etc, so below par essentially means sub standard, and has nothing to do with golf.

If your volunteering is what? If your volunteering is sub-par?

One is not very good at mothering.

The word "submarine" is an example of a word with the prefix "sub."

I have represented a company that would rate higher if you were with a sub par insurance company.

The thought behind this was that some sub par companies would not report their losses to the Comprehensive Loss Underwriting Exchange so they could not verify if you had any insurance claims.